This invention relates to RF circulator devices, and more particularly to four-port circulators.
Circulators are necessary in active array antennas to route RF energy from the transmit/receive ("T/R") modules to the radiating elements and vice versa. Thus, for example, as shown in FIG. 1, a circulator 30 conducts RF energy routed via variable phase shifter 20, variable impedance 22, T/R switch 24 and high power amplifier 26 to the radiating element 32, and from the radiating element 32 to lower noise amplifier 28, and to attenuator 22 and phase shifter 24 via the T/R switch 24.
A four-port circulator has been incorporated into a flared notch radiator, resulting in improved impedance match and isolation, as described in U.S. Pat. No. 5,264,860 entitled "Metal Flared Radiator with Separate Isolated Transmit and Receive Ports", by Clifton Quan, and commonly assigned with the present application.
A conventional method of realizing miniature four-port circulators is to combine two single junction three-port microstrip circulators in a coplanar fashion either side-to-side or end-to-end, as shown in FIGS. 2A and 2B, respectively. Dual junction four-port circulators built in this fashion can be made to operate across a wide (&gt;40%) frequency band. Coplanar integration results in a four-port circulator that is physically larger (wider for the side-to-side configuration; longer for the end-to-end configuration) than the original three-port circulator. These approaches have physical size limitations because of field interaction as the magnets from the two three-port junctions get close together. These size restrains can limit the ability to design antenna lattices that will meet certain radar and radar cross-section (RCS) requirements. Also, these field interactions that occur when the magnets are too close to each other can result in degraded RF circulator performance.
The increase in physical size due to this type of coplanar integration can result in a significant penalty in array depth and weight since potentially thousands of these circulators could be used in a single antenna system. The large size of the dual junction coplanar four-port circulator also limits how compact the antenna array lattice can be which in turn limits the antenna and radar cross-section (RCS) performances.
Single junction four-port circulators have been realized in microstrip. They can be made smaller than the dual junction coplanar four-port circulator, but to date they only operate across a narrower frequency band.
It is therefore an object of this invention to provide a four-port circulator of reduced size than can be achieved using conventional coplanar integration techniques.
It is a further object to provide a four-port circulator formed by two three-port units which operates over a wider frequency band than single junction four-port units.